Organic light emitting devices (OLEDs) are light emitting devices which use organic electroluminescent (EL) materials excited by electric current to emit light. A plurality of OLEDs can be arranged in an array to form a display.
In color displays, it is necessary to arrange OLEDs which emit light of different colors, typically the primary colors blue, green and red, in close proximity to each other. In such an application, it is advantageous to arrange three OLEDs, each emitting light of one of the three primary colors, in a stack, thereby forming a true color pixel from which any color can be emitted. Such a structure is described in PCT International Application WO 96/19792.
In such a stacked structure, one electrode layer is provided at the bottom of the SOLED stack and a further electrode layer is provided between each of the OLEDs in the stack and on top of the uppermost OLED in the stack. The bottom electrode layer is typically coupled to a ground reference and the intermediate and top electrode layers are coupled to either a positive or negative driving voltage.
With only one electrode layer being provided in the SOLED for coupling to a ground reference and with only one additional electrode layer being provided for each OLED in the stack, the problem arises of driving each of the OLEDs in the stack independently of each other. There is therefore a need for a means of driving such a configuration of stacked OLEDs or for alternative configurations of stacked OLEDs.